The invention relates to a method of improving connection quality and system capacity in a cellular radio system comprising in each cell at least one base station communicating with subscriber terminals in its area, the base station transmitting on at least one carrier wave frequency, and in the system a signal transmitted at each frequency being time divided into a frame consisting of a plurality of time slots, and the base stations transmitting information concerning themselves to the subscriber terminals.
The invention also relates to a cellular radio system comprising in each cell at least one base station communicating with subscriber terminals in its area, the base station transmitting on at least one carrier wave frequency, and in the system a signal transmitted at each frequency being time divided into a frame consisting of a plurality of time slots, and the base stations transmitting information concerning themselves to the subscriber terminals.
In a cellular radio system, user speech or data information is transmitted between a base station and a subscriber terminal on a traffic channel. In addition, various control messages and system information are needed between the base station and the subscriber terminal. Control channels are used for transmitting information of this type. In the GSM system, for example, the BCCH channel is used for transmitting connection set-up information from a base station to subscriber terminals. The BCCH channel is used for transmitting cell-specific information. Other channels transmitted in the first time slot, denoted by the number 0, of the BCCH carrier wave frequency are e.g. AGCH and PCH, used for transmitting call set-up information, and SCH and FCCH, used for synchronizing a subscriber terminal.
In the present GSM system, the carrier wave frequency included in a BCCH signal, i.e. the BCCH carrier wave frequency, is transmitted continuously at the same power level. A subscriber terminal continuously measures the power level of BCCH carrier wave frequencies transmitted by adjacent base stations, and reports the measurement results to the base station serving the subscriber terminal. On the basis of these measurement results the system decides a suitable instant for handover to another base station. In order to be able to measure signal attenuation between base stations and subscriber terminals, the BCCH carrier wave frequencies must be transmitted at a constant power level.
In the present GSM system a subscriber terminal has only a very limited time for measuring the power levels of the BCCH carrier wave frequencies of neighboring base stations. If the subscriber terminal receives a signal from the base station in the time slot 0 of a frame, it transmits a signal to the base station in the time slot 3 of the frame and measures neighboring base stations in the time slots 5 and 6 of the frame. Since reception can take place in any of the time slots 0 to 7 of a frame, transmission and base station measurement can also take place in any the time slot of a frame. This is why the BCCH carrier wave frequency must be transmitted continuously.
In the present GSM system, the requirement that the BCCH carrier wave frequency must be sent continuously at a constant power level prevents the use of certain methods developed for improving connection quality and system capacity in low-capacity base stations comprising only one radio unit. The problem is not equally severe in base stations comprising a plurality of radio units, but connection quality and system capacity are impaired even then. Methods for improving connection quality and system capacity in cellular radio systems include e.g. frequency hopping, discontinuous transmission and power control.
It is the object of the present invention to improve connection quality and system capacity in a cellular radio system.
This is achieved with the type of method described in the preamble, characterized in that the subscriber terminal transmits in two time slots and receives in two time slots during one frame, and that the subscriber terminal employs the time slots of the frame following the frame used for transmission and reception for measuring neighboring base stations.
The cellular radio system of the invention is characterized in that the base stations and subscriber terminals comprised by the cellular radio system comprise means for transmitting in two time slots and receiving in two time slots during one frame, and that the subscriber terminals comprise means for employing the time slots of the frame following the frame used for transmission and reception for measuring neighboring base stations.
The method and cellular radio system of the invention provide distinct advantages compared with prior art. Owing to the method and cellular radio system of the invention, methods developed for improving connection quality and system capacity in a cellular radio system, such as frequency hopping, discontinuous transmission and power control, can be used on the traffic channels of the base station BCCH carrier wave frequency. The use of the method of the invention brings forth the above advantages best in low-capacity base stations comprising only one radio unit. The method also improves connection quality and system capacity in base stations comprising a plurality of radio units.
The method and cellular radio system of the invention also provide advantages under circumstances when the BCCH carrier wave frequency is transmitted continuously at a constant power level. Under such circumstances the subscriber terminals can perform more measurements in a time unit than in the present system. This again assists the system in deciding the suitable instant for handover to another base station, and consequently improves connection quality.
Frequency hopping refers to the transmission frequency used on a connection being changed at predetermined intervals. By frequency hopping, transmission quality can be improved particularly in cases when a subscriber terminal is moving very slowly or is stationary, as is often the case when a hand-held mobile telephone is used. Frequency hopping also helps in dividing the interference caused by the radio connection to several frequencies, and this results in a momentary disturbance at a given frequency remaining small.
Discontinuous transmission again is an operational state in which the transmitter is switched off for the time when there is no data to be transmitted. This contributes to reducing the interference caused by the radio connection and improves the reuse of frequencies, resulting in improved total system capacity.
Because the transmission level of a subscriber terminal and a base station can be controlled, a situation in which the transmission power of a cell would have to be increased so as to exceed the power needed in normal areas because of shadow areas in order to provide a satisfactory field strength for the shadow areas can be avoided. Such a situation would lead to the field strength being unnecessarily high in normal areas leading to impaired reuse of frequencies and consequently to reduced total system capacity.